1,2-Polybutadiene having crystallinity has hitherto been obtained by a catalyst comprising a phosphine complex of a cobalt salt, a trialkylaluminum and water (patent document 1: JP-B-44-32425, patent document 2: JP-A-1-249788), a catalyst comprising a phosphine complex of a cobalt salt and methylaluminoxane (patent document 3: JP-A-8-59733), or a catalyst comprising a cobalt salt slurry, a phosphine solution and methylaluminoxane [non-patent document 1: Dilip C. D. Nath, Takeshi Shiono and Tomiki Ikeda, Journal of Polymer Science, Vol. 40, 3086–3092 (2002)].
In the catalyst systems described these, it can be known by analogy that a phosphine compound having three aromatic groups is substantially necessary in the production of a 1,2-polybutadiene-based polymer having a crystallinity of 34% or more. However, when those phosphine compounds are used, it is required to lower the polymerization temperature, which causes an increase in the amount of a solvent used for preventing precipitation, and in the production of the 1,2-polybutadiene-based polymer obtained by an exothermic reaction, the problem of increasing energy loss such as the necessity for higher cooling capacity to a polymerization reactor.
In the above-mentioned patent document 1 (JP-B4432425), there is described an example to use diphenylethylphosphine as a phosphine compound having one aliphatic group and two aromatic groups. However, it is described that an amorphous (that is to say, having a crystallinity of 0%) polymer is obtained when the phosphine compound is used. It is difficult to know the production of a 1,2-polybutadiene-based polymer having a crystallinity of 34% or more by analogy with the category of phosphines as specifically exemplified in the gazette concerned, that is to say, the phosphine compound having one aliphatic group and two aromatic groups.
Further, in the catalyst systems described in these, when the phosphine complex of the cobalt salt is used, equipment for preparing a catalyst component becomes large to necessitate excessive investment, because of low solubility of the phosphine complex in an organic solvent. Furthermore, when a cobalt chloride slurry is used, the efficiency of catalyst is low. It is therefore necessary to use a large amount of catalyst in order to obtain a polymer, which causes the problems of the coloration of the polymer due to residual catalyst and the like.
The present invention relates to a process for producing 1,2-polybutadiene having a high crystallinity using a specific catalyst system, wherein the vinyl bond content of a polymer obtained is high, and the molecular weight thereof is adjustable.